Centrifugal molding of ceramic tubes containing metal fibers

ABSTRACT

A sound strong ceramic body of tubular or pipe shape is obtained by centrifugally casting a mixture of ceramic powders and fibrous metal, the metal having a coefficient of expansion higher than that of the ceramic.

CENTRIFUGAL MOLDING OF CERAMIC TUBES CONTAINING METAL FIBERS Thisapplication is a continuation-in-part of application Ser. No. 719,977,filed Apr. 9, 1968, and now abandoned.

This invention relates to the production of ceramic tubes reinforcedwith fibrous metal and in particular large tubes having a diameter of 6to 8 inches, a wall thickness of one-half inch and a length upto l feet.

Transmission of fluids at high temperatures, particularly in theinstance of the petrochemical industry, demands tubing of suchcomposition as to be resistant to both the high temperatures involvedand the corrosive atmosphere prevailing. The mechanical stresses thatmay be imposed also demand strong tubes, both from the standpoint ofextermal loading and pressures that prevail internally.

A ceramic tube will satisfy the demand of resistance to heat andcorrosive influence, but ceramic products are susceptible to brittlefailure. A mere fissure or surface defect per se could be tolerated in acermic tube as involving at the worst a detectable leak, but there canbe no allowance in overhead systems for catastrophic failure understress or impact loading.

The primary object of the present invention is to develop a compositeceramic tube presenting acceptable strength from the standpoint ofresistance to external and internal loading and to accomplish this bycasting the tube centrifugally from a mixture of a ceramic and metalfibers. Another object of the present invention is to pre-stress theceramic matrix by selecting the fibers of a metal having an expansioncoefficient greater than that of the ceramic.

In the practice of the present invention, any finely divided ceramic maybe used in conjunction with fibrous metal having the necessarycoefficient of expansion. We define a ceramic as any inorganicnon-metallic such as to include silica, alumina, titania, zircon,silicon carbide and so on. The requirements are basically physical, notchemical, and many equivalents can obviously be used. The only otherlimitation on the metal is that its melting point be above thetemperature at which the tube will be cured and the temperature to whichit will be exposed in actual service. For most purposes, any ferro-alloyor nickel-chromium alloy (IN- CONEL or Nl-CHROME) may be used. However,it should be distinctly understood that for severe service conditions wecan also use ceramics such as aluminum silicate (e.g., mullite) andzirconium silicate for the ceramic matrix, and highly refractory metalssuch as tungsten and molybdenum.

Other and further objects of the present invention will be apparent fromthe following description and claims and are illustrated in theaccompanying drawings, disclosing the preferred embodiment of thepresent invention and the principle thereof and what we now consider tobe the best mode contemplated for applying these principles. Otherembodiments of the invention embodying the same or equivalent principlesmay be used and changes may be made as desired by those skilled in theart without departing from the present invention.

In the drawings:

F IG. 1 is a view in perspective of centrifugal molding equipment;

FIG. 2 and FIG. 3 are cross sections of tubing molded in accordance withthe present invention; and

FIGS. 4, 5, and 6 are cross sections of tubing.

Under one mode of practicing the present invention, a ceramic slurry isprepared and molded centrifugally while incorporating metal fibers inthe slurry before or during molding. While any castable ceramic can beused, the metal fibers, to develop superior strength, must have a highercoeicient of expansion than the ceramic. If this precaution is notobserved, cracking may occur in the molded tube'due to residual stressesin the ceramic occurring during the cooldown period from the firingtemperature used to cure or harden (sinter) the ceramic. If theexpansion phenomenon is observed, a form of reinforcement is introducedinto the ceramic matrix so that we develop high resistance to crackingdue to impact.

The slurry containing the metal fibers is introduced into a circularmetal mold rotating at high speed. The mixture spreads itself withuniform thickness on the mold wall due to centrifugal force developed inthe spinning mold. Free water gravitates toward the axis of the moldwhere it is easily drained, while the metal fibers are forced toward theouter limit of the spun tube where they align themselves uniformlyconcentric to the wall of the cast tube. After the cast is completed,the green tube is stripped from the mold, dried in air, and fired toharden the same.

Tests demonstrate a strength almost five times greater, with only about5 percent (volume) metal fiber addition, compared to a tube solely ofthe ceramic. The metal fiber content, however, may be as low as about 3percent, but no advantages are realized when the metal fiber contentexceeds 30 percent by volume (dry basis).

Diametrical compression under expected loads may produce fissures, butnot disintegration, and any cracks or fissures which do appear close upwhen the load is removed, manifesting an unexpected advantage whichappears to be one of pre-stressing. The present invention isparticularly suited to the production of strong, thin-walled cylindersimpossible to produce by extrusion techniques.

The work hereinafter described in detail was directed toward themanufacture of cast tubes 6 to 8 inches in diameter with a wallthickness of one-half inch and at least 10 feet long, with expectedoperating conditions of almost 2,700F and gas flow of 2,500 feet persecond under an internal pressure of 40 pounds per square inch (gauge).

While a castable slip may be used, one eminently satisfactory system isan aqueous slurry of' fused silica containing chopped stainless steelfibers 0.003 inches diameter and from one-eighth to one-half inch inlength. The metal fibers preferably have a length greater than l0 timesthe diameter. However, we are not limited to such specifications,especially since the water content serves only to expedite compaction byreducing the centrifugal force necessary to produce compaction. Thus,the water content may be progressively less with progressively highercentrifugal forces during molding. The specific materials, silica andstainless steel, present the prerequisites of low elastic modulus forthe matrix, high strength and high elastic modulus for the metal fibers,and a coefficient of expansion for the metal higher than that of theceramic:

TABLE l Fused Property Silica Stainless Steel Coefficient of expansion(in,-/in./F) 1.0 X 10-6 8.2 10-6 Elastic Modulus (Young, lpsi) 10.5 28.0

A series of experiments were initially performed without fibers toevaluate the feasibility of centrifugally casting a ceramic slurry,using the equipment illustrated in FIG. 1 typical of centrifugal castingapparatus wherein a cylindrical mold of tubular construction 20 issecured to a rotatable hub 21 in turn driven by a motor 22. The end ofthe centrifugal mold opposite the hub is suitably meshed with a ringgear 23 which establishes uniformity of rotation. In this initialfeasibility determination, the castable ceramic slurry was introducedinto the mold through a manually manipulated conduit, and the conduitwas moved along the axis of the mold interiorally thereof to spread theslurry along the length of the mold. The mold was rotated at 1,500-2,000rpm and of course the mixture spread itself with uniform thickness. Thespinning forces cause free water to gravitate to the inside surface ofthe casting where it is free to drain, and in this connection it will beappreciated that after the tube is formed the mold may be tippedslightly to allow the water to drain by gravity.

From the initial determination, it was established that a centrifugalmold is indeed capable of spin casting a ceramic slurry to produce asound, dense tube that can be hardened at high temperature. It was alsodetermined that removal ofthe casting is facilitated by using a splitmold (two 180 segments) having quick release capabilities and that theaddition of ball clay to the ceramic mixture will produce shrinkage ofthe casting after standing for some time, contributing to easyseparation of the casting from the centrifugal mold. We can useTennessee No. 5 ball clay, or any equivalent form.

Further experimental investigations, using stainless steel fibersone-half inch long and 0.003 inches diameter, established that metalfibers cannot be easily mixed into the ceramic slurry; they matted forthe most part into a ball, and spin casting could not be accomplished.This difficulty was overcome by introducing the slurry separately intothe mold with the metal fibers thereafter introduced into the ceramicmatrix while spinning the mold with the slurry in a wet state. The metalfibers were introduced into the slurry in the spinning mold simply byfeeding the metal fibers through a conduit while maneuvering thedischarge and of the conduit to produce uniform sprinkling of the metalfibers.

The silica, ball clay, and water are mixed to a homogeneous stateoutside the mold and then introduced as above described into thecentrifugal mold spinning only at a speed sufficient to spread theslurry uniformly outward along the length of the mold, say about 500rpm. The metal fibers are thereafter introduced into the slurry whilecontinuing to spin the mold at about the same speed. The fibers may beso introduced by spilling them from the discharge end of a conduit,moving from one end of the mold to the other, the rates being such as toproduce a substantially uniform sprinkling of the metal fibers onto theexposed surface of the slurry.

Thereafter the mold is spun at about 1,800 rpm for about 6 minutes,expunging nearly all the water and causing the metal fibers to arrangethemselves concentric to the wall of the cast tube throughout itsthickness. The mold halves are disassembled toexpose the green tubewhich is strong enough to be handled, the tube being allowed to dry inair for 24 hours. Thereafter the green tube is fired, preferably in anon-oxidizing atmosphere (e.g., percent nitrogen, 5 percent hydrogen) toavoid oxidation of the steel fibers. To inhibit promotion oftransformation products of the ceramic content the firing temperatureshould not exceed 2,000 F. The air-dried tube is slowly brought up totemperature, starting at room temperature, in increments of 250 F perhour, requiring 8 hours; the tube is then held at 2,000 F for 3 to 10minutes, whereafter it is removed from the furnace and allowed to coolin air overnight, or at about F 1 hour. Other schedules may be usedsince there is no criticality in this regard.

If desired, the metal fibers may be mixed with the slurry beforeintroducing the slurry into the mold as above described, but if this isdone then a small amount of a binder should be employed in order toprevent matting of the metal fibers when mixing. The binder may be about3 to 5 grams of bentonite or as little as half a gram of methylcellulose, or a mixture of the two.

During firing, which cures and hardens the tube, manifest in sinteringof the ceramic particles, there is some diffusion bonding between themetal and the ceramic. The metal, during cooling, will shrink morerapidly than the ceramic; the metal goes into tension, pulling theceramic into compression and the latter becomes pre-compressed.

Tubes prepared from a ceramic slurry alone, without metal fibers, andotherwise processed almost exactly like the tubes of Example l showedmuch less strength under diametral compression loading, namely, anaverage stress of 245 psi at failure compared to an average of 1,030 psiat failure for tubes made in accordance with Example I. Thus, the tubesfor comparison had the following composition: *325 mesh fused silica,3,160 grams (98.8 vol. percent); ball clay (Tennessee No. 5), 50 grams(1.2 vol. percent) and 420 grams water, centrifugally cast and fired asspecified under Example l.

FIGS. 2 and 3 are perspective views of shapes contemplated under thepresent invention and serving as comparisons for what is shown in FIGS.4, 5, and` 6. Thus, evidence of the benefits of reinforcement isexhibited in FIG. 4. A green or unfired section of an Example l pipejust separated from the mold was accidentally dropped. Instead ofshattering into small pieces, only the edge where the pipe hit the floorcracked as shown. Even here the broken pieces could not easily bedetached from the pipe section. While this accident was not intentional,it does indicate the improved toughness of the spin-cast reinforcedpipe.

Additional evidence of improved behavior is shown in FIGS. 5 and 6.Unreinforced pipe sections will fail in four sections under diametralcompressive loading. The fiber reinforced section cracked, but did notseparate into sections. In fact, when the load was released the sectionsresumed their original circular shape, FIG` 5. FIG. 6 is a close-up ofthe crack which does not penetrate to the inside diameter of the pipesection, establishing that pre-stress has been introduced due to themetal having a higher coefficient of expansion than the vceramic matrix.The parts shown in FIGS. 5 and 6 were cast and fired in accordance withExample I.

There is nothing critical about spinning speed, so long as it is highenough to throw the slurry against the mold wall. Thus, spinning speedonly determines (inversely) how long the mold must be rotated to alignthe fibers and expunge enough water to produce a dense tube which can beeasily handled when it is removed from the mold. Likewise, there isnothing critical about air drying the tube removed from the mold sincethe purpose is to have the tube dry enough so that it will not crackwhen subjected to the firing schedule.

Hence while we have illustrated and described a preferred embodiment ofthe invention, it is to be understood that this is capable of variationand modification.

We claim:

1. A method of producing a ceramic tube reinforced by metal andcomprising: presenting to the interior of a centrifugal mold ofcylindrical form a mixture of ceramic particles and metal fibersdispersed in water, said mixture including a binder selected from thegroup consisting of bentonite and methyl cellulose in an amountsufficient to prevent matting of the metal fibers in the mixture, themetal fibers having a coefficient of expansion higher than that of theceramic and a length greater than ten times their diameter, and saidmold consisting essentially of a tubular member having an insidediameter representing the outside diameter of a green ceramic tube to beformed therein; rotating the mold to cause the metal fibers uniformly toalign themselves concentric to the wall of the centrifugal mold and toseparate water from said mixture, producing a green ceramic tubecontaining the fibers; draining the water from the mold and removing thegreen tube from the mold, drying the green tube in air,l and thenheating the tube to sinter the ceramic particles as a matrix surroundingthe metal fibers, the metal melting above sintering temperature.

2. A method according to claim' l in which the ceramic is silica and themetal is stainless steel, the metal fibers being present in an amount ofat least about 3 percent but not more than about 30 percent by volumebased on the ceramic and the metal.

3. A method according to claim 2 in which the parameter for molding isabout 1,800 rpm for about 6 minutes, and in which the air-dried tube isbrought gradually up to a temperature of about 2,000F where it is heldfor about 3 to l0 minutes and then allowed to cool in air.

4. A method of producing a ceramic tube reinforced by metal andcomprising: presenting to the interior of a centrifugal rn ld ofcylindrical form a mixture of ceramic partic es dispersed in water, saidmold consisting essentially of a tubular member having an insidediameter representing the outside diameter of a green ceramic tube to beformed therein; rotating the mold containing said mixture and while themixture is being so rotated adding metal fibers thereto, the metalfibers having a coefficient of expansion higher than that of the ceramicand a length greater than ten times their diameter, rotation of the moldcausing the metal fibers uniformly to align themselves concentric to thewall of the centrifugal mold and causing water to separate from saidmixture resulting in a green ceramic tube containing the fibers;draining the water from the mold and removing the green tube from themold, drying the green tube in air, and then heating the green tube tosinter the ceramic particles as a matrix surrounding the metal fibers,the metal melting above sintering temperature.

5. A method according to claim 4 in which the ceramic is silica and themetal is stainless steel, the metal fibers being present in an amount ofat least about 3 percent but not more than about 30 percent by volumebased on the ceramic and the metal.

6. A method according to claim 5 in which the parameter for molding isabout 1,800 rpm for about 6 minutes, and in which the air-dried tube isbrought gradually up to a temperature of about 2,000F where it is heldfor about 3 to l() minutes and then allowed to cool in air.

2. A method according to claim 1 in which the ceramic is silica and themetal is stainless steel, the metal fibers being present in an amount ofat least about 3 percent but not more than about 30 percent by volumebased on the ceramic and the metal.
 3. A method according to claim 2 inwhich the parameter for molding is about 1,800 rpm for about 6 minutes,and in which the air-dried tube is brought gradually up to a temperatureof about 2,000*F where it is held for about 3 to 10 minutes and thenallowed to cool in air.
 4. A method of producing a ceramic tubereinforced by metal and comprising: presenting to the interior of acentrifugal mold of cylindrical form a mixture of ceramic particlesdispersed in water, said mold consisting essentially of a tubular memberhaving an inside diameter representing the outside diameter of a greenceramic tube to be formed therein; rotating the mold containing saidmixture and while the mixture is being so rotated adding metal fibersthereto, the metal fibers having a coefficient of expansion higher thanthat of the ceramic and a length greater than ten times their diameter,rotation of the mold causing the metal fibers uniformly to alignthemselves concentric to the wall of the centrifugal mold and causingwater to separate from said mixture resulting in a green ceramic tubecontaining the fibers; draining the water from the mold and removing thegreen tube from the mold, drying the green tube in air, and then heatingthe green tube to sinter the ceramic particles as a matrix surroundingthe metal fibers, the metal melting above sintering temperature.
 5. Amethod according to claim 4 in which the ceramic is silica and the metalis stainless steel, the metal fibers being present in an amount of atleast about 3 percent but not more than about 30 percent by volume basedon the ceramic and the metal.
 6. A method according to claim 5 in whichthe parameter for molding is about 1,800 rpm for about 6 minutes, and inwhich the air-dried tube is brought gradually up to a temperature ofabout 2,000*F where it is held for about 3 to 10 minutes and thenallowed to cool in air.